System and method for screening a library of samples

ABSTRACT

A continuous throughput microfluidic system includes an input system configured to provide a sequential stream of sample plugs; a droplet generator arranged in fluid connection with the input system to receive the sequential stream of sample plugs and configured to provide an output stream of droplets; a droplet treatment system arranged in fluid connection with the droplet generator to receive the output stream of droplets in a sequential order and configured to provide a stream of treated droplets in the sequential order; a detection system arranged to obtain detection signals from the treated droplets in the sequential order; a control system configured to communicate with the input system, the droplet generator, and the droplet treatment system; and a data processing and storage system configured to communicate with the control system and the detection system.

CROSS-REFERENCE OF RELATED APPLICATION

This application claims priority to U.S. Provisional Application Nos.61/567,837 filed Dec. 7, 2011; 61/638,241 filed Apr. 25, 2012;61/638,245 filed Apr. 25, 2012; the entire contents of all of which arehereby incorporated by reference.

This invention was made with Government support of Grant No. R21CA120742, awarded by the Department of Health and Human Services, TheNational Institutes of Health (NIH). The U.S. Government has certainrights in this invention.

BACKGROUND

1. Field of Invention

The field of the currently claimed embodiments of this invention relatesto microfluidic systems, and more particularly to continuous throughputmicrofluidic systems.

2. Discussion of Related Art

High throughput sample processing is a critical requirement for a largenumber of industries. Some examples include the agricultural,pharmaceutical and biotechnological industries¹. As a result, there is aconstant drive for innovation in sample processing techniques to supportthese industries. One major breakthrough in this domain has been theapplication of various robotic sample handling techniques, to improvethe speed of sample processing as well as to reduce the volume ofreagents used per reaction. Although the robotic systems have becomeincredibly fast at sample processing operations, they are typicallylimited to operating with standard multi-well (96, 384 and 1536 well)plates. As a result, the typical sample volume consumption is on theorder of microliters per reaction for such systems¹. Recent advances inthe microfluidic domain show promise in overcoming this limitation ofthe robotic systems. Droplet-based microfluidic systems have been shownto be capable of performing biomolecular screening with sample volumesas low as picoliters²⁻⁶. However, introducing a large number of sampleson a miniature microfluidic device is difficult since it is impracticalto have hundreds to thousands of sample inlets to a single microfluidicdevice. Furthermore, the tubing used for supplying the samples to such amicrofluidic device would already consume orders of magnitude moresample than is required for the actual analysis on the microfluidicdevice. So, there is a need for an efficient way to transport a largenumber of samples to a microfluidic device. Ideally such a sampletransport system would be flexible enough to supply variable number ofsamples to a microfluidic device without any modifications in thetransport system or the device.

The ‘plug-in cartridge’ technique developed by the Whitesides group⁷provides an elegant solution to the problem of introducing a largenumber of reagents on a microfluidic device through a single inlet.Under this approach, a series of sample plugs are loaded into acapillary, with air bubbles present between sample plugs acting asspacers. This capillary is connected to a microfluidic device, forserial delivery of these sample plugs. However, in this approach, thesample plugs are constantly in contact with the capillary inner surface,leading to the problem of cross contamination between plugs⁷. Anothermodification of this approach developed by the Ismagilov group⁸ utilizesan immiscible carrier fluid instead of an air bubble to act as a spacerbetween sample plugs. The carrier fluid in this approach preferentiallywets the inner surface of the capillary, thus preventing direct contactbetween sample plugs and the capillary surface. As a result, the problemof cross contamination between sample plugs is eliminated. The carrierfluids typically used for generating these sample plug arrays arefluorinated oils, which also reduce the problem of reagents leaking fromsample plugs into the carrier fluid due to their low solubility for mostreagents⁸.

Although this approach is promising, the current techniques used underthis approach for generating the ‘sample plug cartridges’ have someissues which need to be resolved. The common technique of using asyringe pump for aspirating sample plugs from a sample well⁹⁻¹¹ in amulti-well plate can be extremely slow. Another technique of usingvacuum for aspirating a sample plug can be much faster⁷. However, thistechnique can only provide a maximum driving pressure of 1 atm (˜15psi). As a result, the driving force may not be sufficient to load largenumbers of sample plugs into a capillary due to the increasing fluidicresistance of the capillary with the introduction of sample plugs.Furthermore, both of these techniques require the free end of thecapillary to be attached to either a syringe or a vacuum source, thusexcluding the possibility of operating this sample loading system insync with the operations on a downstream microfluidic device. This canbe a major setback to throughput as the possibility of conducting assaysin continuous flow manner on microfluidic devices, as has beendemonstrated earlier¹², is precluded. Therefore, there remains a needfor improved systems and methods for screening large libraries ofsamples.

REFERENCES FOR BACKGROUND SECTION

-   -   (1) Mayr, L. M.; Bojanic, D. Novel trends in high-throughput        screening Curr. Opin. Pharmacol. 2009, 9, 580-588.    -   (2) Teh, S. Y.; Lin, R.; Hung, L. H.; Lee, A. P. Droplet        microfluidics Lab. Chip 2008, 8, 198-220.    -   (3) Rane, T. D.; Puleo, C. M.; Liu, K. J.; Zhang, Y.; Lee, A.        P.; Wang, T. H. Counting single molecules in sub-nanolitre        droplets Lab. Chip 2010, 10, 161-164.    -   (4) Brouzes, E.; Medkova, M.; Savenelli, N.; Marran, D.;        Twardowski, M.; Hutchison, J. B.; Rothberg, J. M.; Link, D. R.;        Perrimon, N.; Samuels, M. L. Droplet microfluidic technology for        single-cell high-throughput screening Proc. Natl. Acad. Sci.        U.S.A. 2009, 106, 14195-14200.    -   (5) Kiss, M. M.; Ortoleva-Donnelly, L.; Beer, N. R.; Warner, J.;        Bailey, C. G.; Colston, B. W.; Rothberg, J. M.; Link, D. R.;        Leamon, J. H. High-throughput quantitative polymerase chain        reaction in picoliter droplets Anal. Chem. 2008, 80, 8975-8981.    -   (6) Zhong, Q.; Bhattacharya, S.; Kotsopoulos, S.; Olson, J.;        Taly, V.; Griffiths, A. D,; Link, D. R.; Larson, J. W. Multiplex        digital PCR: breaking the one target per color barrier of        quantitative PCR Lab. Chip 2011, 11, 2167-2174.    -   (7) Linder, V.; Sia, S. K.; Whitesides, G. M. Reagent-loaded        cartridges for valveless and automated fluid delivery in        microfluidic devices Anal. Chem. 2005, 77, 64-71.    -   (8) Chen, D. L.; Ismagilov, R. F. Microfluidic cartridges        preloaded with nanoliter plugs of reagents: an alternative to        96-well plates for screening Curr. Opin. Chem. Biol. 2006, 10,        226-231.    -   (9) Zheng, B.; Ismagilov, R. F. A microfluidic approach for        screening submicroliter volumes against multiple reagents by        using preformed arrays of nanoliter plugs in a three-phase        liquid/liquid/gas flow Angew. Chem. Int. Ed Engl. 2005, 44,        2520-2523.    -   (10) Adamson, D. N.; Mustafi, D.; Zhang, J. X.; Zheng, B.;        Ismagilov, R. F. Production of arrays of chemically distinct        nanolitre plugs via repeated splitting in microfluidic devices        Lab. Chip 2006, 6, 1178-1186.    -   (11) Li, L.; Mustafi, D.; Fu, Q.; Tereshko, V.; Chen, D. L.;        Tice, J. D.; Ismagilov, R. F. Nanoliter microfluidic hybrid        method for simultaneous screening and optimization validated        with crystallization of membrane proteins Proc. Natl. Acad. Sci.        U.S.A. 2006, 103, 19243-19248.    -   (12) Zhang, Y.; Ozdemir, P. Microfluidic DNA amplification—a        review Anal. Chim. Acta 2009, 638, 115-125.    -   (13) Abramoff, M. D.; Magalhaes, P. J.; Ram, S. J. Image        Processing with ImageJ Biophotonics International 2004, 11,        36-42.

SUMMARY

A continuous throughput microfluidic system according to an embodimentof the current invention includes an input system configured to providea sequential stream of sample plugs; a droplet generator arranged influid connection with the input system to receive the sequential streamof sample plugs and configured to provide an output stream of droplets;a droplet treatment system arranged in fluid connection with the dropletgenerator to receive the output stream of droplets in a sequential orderand configured to provide a stream of treated droplets in the sequentialorder; a detection system arranged to obtain detection signals from thetreated droplets in the sequential order; a control system configured tocommunicate with the input system, the droplet generator, and thedroplet treatment system; and a data processing and storage systemconfigured to communicate with the control system and the detectionsystem. The control system is configured to control the input system inconjunction with the droplet generator and to provide information to thedata processing and storage system that identifies each droplet of theoutput stream of droplets with a corresponding sample plug of thesequential stream of sample plugs. The control system further controlsthe droplet treatment system and provides information to the dataprocessing and storage system that identifies a treatment applied toeach droplet of the output stream of droplets, and the data processingand storage system receives the detection signals and calculates aproperty of each treated droplet and identifies a corresponding plug andtreatment for each treated droplet based on the sequential order.

A method of screening a plurality of samples according to an embodimentof the current invention includes obtaining a sequential stream ofsample plugs, generating a sequential stream of droplets from thesequential stream of sample plugs using a selectively controllablemicrofluidic system, adding at least one reagent from a plurality ofreagents to each of the sequential stream of droplets in sequentialorder using the selectively controllable microfluidic system, andmeasuring at least one physical property of each of the sequentialstream of droplets in the sequential order. Information concerning anidentity of the plug from which each droplet of the sequential stream ofdroplets is generated and the at least one reagent added to each dropletis used to identify measured droplets based on the sequential orderduring the measuring.

A continuous throughput microfluidic system according to an embodimentof the current invention includes an input system configured to providea sequential stream of sample plugs; a droplet generator arranged influid connection with the input system to receive the sequential streamof sample plugs and configured to provide an output stream of droplets;a droplet treatment system arranged in fluid connection with the dropletgenerator to receive the output stream of droplets in a sequential orderand configured to provide a stream of treated droplets in the sequentialorder; a detection system arranged to obtain detection signals from thetreated droplets in the sequential order; a control system configured tocommunicate with the input system, the droplet generator, and thedroplet treatment system; and a data processing and storage systemconfigured to communicate with the control system and the detectionsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objectives and advantages will become apparent from aconsideration of the description, drawings, and examples.

FIG. 1 is a schematic illustration of a continuous throughputmicrofluidic system according to an embodiment of the current invention.

FIG. 2 is a schematic illustration of the sample screening platform ofthe embodiment of FIG. 1. Step 1: A cartridge is loaded with a libraryof sample plugs separated by an immiscible carrier fluid. This cartridgeis interfaced with a microfluidic device. Step 2: On-demand digitizationof incoming sample plugs into smaller daughter droplets. The volume ofindividual daughter droplets can be controlled by valve opening time andback pressure on the cartridge. Step 3: By exploiting the crosssectional area of the central channel, the sample daughter droplet isstretched in the “Fusion Region”. This approach allows for robust,synchronization-free injection of up to four reagents simultaneouslydirectly into the daughter droplet. The volume of reagent injected iscontrolled through modulation of back pressure and valve opening timecorresponding to the reagent inlet. Step 4: Once sample and reagent havebeen combined into one droplet, the droplets are stabilized with carrierfluid containing surfactant to prevent unwanted droplet coalescencedownstream. Step 5: Sample-reagent droplets travel to an incubationchannel. Step 6: The sample-reagent droplets are incubated whilemaintaining their sequence in downstream incubation channels. Thisapproach allows for droplet identification through spatial indexing in a1-Dimensional array.

FIGS. 3A and 3B illustrate a ‘Serial Sample Loading’ (SSL) systemaccording to an embodiment of the current invention. FIG. 3A; a)Schematic: A custom SSL system was designed which employs positivepressure to inject a sample plug into a microcapillary from a standardmulti-well plate. A custom capillary adapter in the SSL system providesan interface between a microcapillary and a multi-well plate. Thisadapter can seal a well on the multi-well plate, thus generating atemporary pressure chamber inside the sample well. A pressure input onthe adapter can then be used to apply controlled pressure to the fluidinside the sample well. This positive pressure drives a small plug ofsample from the well into the microcapillary. Following this, the sealis broken and the multi-well plate is moved to seal another sample wellwith the capillary adapter. This sequence of steps is repeated togenerate a sample library into the microcapillary. FIG. 3B: b) An imageof a food dye sample plug array generated using the SSL system. Eachsample plug is separated from each other by an immiscible carrier fluid.

FIGS. 4A and 4B show an example of a microfluidic chip according to anembodiment of the current invention, a) Photograph of a prototypedevice. The microfluidic device has a multichannel architecture: 1) Thecentral channel with fusion region and incubation region (purple), 2)Capillary inlet, 3) Reagent inlets: reagent 1 (pink), reagent 2(orange), reagent 3 (green), reagent 4 (turquoise) and surfactant oilinlet (yellow). The valves on the device (V1-V7) are indicated by aturquoise dye. b) A scan of the capillary inlet region, indicating theheight difference between different sections of the capillary inlet tofacilitate smooth sample plug transition from the large ID of thecapillary to the shallow channels on the microfluidic device.

FIGS. 5A and 5B provide results for control of droplet volume anddroplet uniformity using mechanical valve based droplet generation. a)Micrograph of the incubation region on the microfluidic device filledwith reagent droplets generated using sequentially increasing valveopening times (T_(on)=0.3, 0.4, 0.5, 0.6, 0.7, 0.8 seconds) for a fixedback pressure on the reagent inlet (P_(reagent)=5 psi). This resulted ina linear array of repeats of a sequence of six droplets, with eachdroplet increasing in volume. b) A plot of droplet volume versus thevalve opening time (T_(on)) for a valve corresponding to a reagent inletfor three different values of back pressures applied to the reagentinlet (P_(reagent)=2.5, 5 and 7.5 psi). The droplet volumes plotted arean average of volumes estimated from 50 droplets for each condition. Theerror bars in the volume data are too small to be seen on the plot.

FIGS. 6A and 6B provide a demonstration of reagent injection in sampledaughter droplets merging with reagents. a) Time series of imagesindicating reagent injection into sample daughter droplets at the‘Fusion zone’ on the device. A sample daughter droplet (yellow) isreleased from the capillary inlet and is halted in the ‘Fusion zone’ byactuating a valve upstream which controls carrier fluid injection intothe central channel on the device. A reagent (blue) is released andinjected directly into the sample droplet. The elongated configurationof the sample daughter droplet in the ‘Fusion zone’ ensures robustreagent injection operation on the device without the need for precisesample droplet positioning. b) A series of photographs demonstratinginjection of different numbers of reagents into a sample daughterdroplet simultaneously.

FIGS. 7A and 7B show photographs of the incubation region indicatingmultiplexing capability of the device, a) Table with micrographsindicating combinatorial mixture droplets generated on the device usingfour different samples [Sample A (blue), Sample B (yellow), Sample C(green), Sample D (water)] and four different reagents [Reagent 1(orange), Reagent 2 (water), Reagent 3 (light, blue), Reagent 4(yellow)]. Droplets 1A-4A, 1B-4B, 1C-4C, and 1D-4D are each generatedfrom a combination of the sample and reagent in the corresponding columnand row respectively. For example, Droplet 1A is the combination ofsample A with reagent 1. b) Left panel shows four different micrographsshowing repeating sequences of sample daughter droplets generated from asingle sample plug merged with four different reagents on a singledevice. The right panel shows zoomed in version of a small section ofthe micrographs from the left panel for easy visualization of thedroplet sequence. The droplet identification codes in this panel are thesame as those used in subfigure a. Note: The sequence of droplets seemsto be going in opposite direction in alternate channels due to thechanging direction of flow in serpentine channels.

FIG. 8 illustrates a microfabrication process with single developingstep. Four consecutive layers of SU8 photoresist are spin coated andpatterned on a single silicon wafer using photolithography. Each layerundergoes all standard photolithography steps like soft bake (SB),exposure and post-exposure bake (PEB). However, the developing step isconducted in common for all layers after patterning the last photoresistlayer.

FIG. 9 is an illustration of a modified microfluidic device fabrication:The process flow used for fabrication of our microfluidic devices toaccommodate tall channels in the fluidic layer while maintaining thefunctionality of push-down valves. (6:1, 10:1 and 15:1 refer to theratio of the base to curing agent used to mix a batch of PDMS)

FIG. 10 shows a fusion zone design for robust reagent injection insample droplets with different volumes according to an embodiment of thecurrent invention.

FIGS. 11A and 11B shows an example of a custom Serial Sample Loading(SSL) System according to an embodiment of the current invention. FIG.11A: a) Picture of the custom SSL system. The SSL system features: 1) acustom-made Capillary Adapter, 2) an automated Z-stage, 3) a 96-wellplate, 4) Manual X and Y stages and 5) an Electronic PressureController. FIG. 11B: b) A fluorescence image of sample plugs containingLamda DNA stained with PicoGreen. These sample plugs were generated in asilica microcapillary using the SSL system, without any visual feedbackduring plug generation due to transparent nature of the sample. Theimage indicates high uniformity of the sample plugs despite lack ofvisual feedback: The lack of fluorescence background in the area betweensample plugs (carrier fluid) also indicates minimal sticking of sampleto the inner wall of the capillary preventing chances of crosscontamination between sample plugs. c) A plot of sample plug volumeversus duration of peak pressure application (T_(peak)) at constant peakpressure, (P_(peak): 1 psi). The linear relationship between sample plugvolume and T_(peak) indicates the capability of our SSL system to varysample plug volume in a predictable manner. The small error bars alsoindicate the uniformity of the sample plugs generated for identicalloading conditions.

DETAILED DESCRIPTION

Some embodiments of the current invention are discussed in detail below.In describing embodiments, specific terminology is employed for the sakeof clarity. However, the invention is not intended to be limited to thespecific terminology so selected. A person skilled in the relevant artwill recognize that other equivalent components can be employed andother methods developed without departing from the broad concepts of thecurrent invention. All references cited anywhere in this specification,including the Background and Detailed Description sections, areincorporated by reference as if each had been individually incorporated.

The aforementioned droplet platforms do not address the needs ofnumerous applications which require high degrees of multiplexing as wellas high-throughput analysis of multiple samples. Some examples include,but are not limited to, genetic fingerprinting for forensics¹⁴, singlenucleotide polymorphism (SNP) analysis for crop improvement anddomestication¹⁵, genotyping required for identification of genesassociated with common diseases⁶ and generation of a blood donorgenotype database for better matching between recipient and donor toprevent adverse transfusion reactions¹⁷. All of these applicationsrequire multiplexed screening of a single sample with a panel ofreagents (or markers) and rapid screening of a large number of samplesto generate the required databases.

In recent years, there have been attempts to expand the capacity ofdroplet platforms for the analysis of a biological or chemical samplewith multiple reagents. One of the well-tested platforms has been thedroplet platform developed by RainDance Technologies, for massivelyparallel PCR enrichment for DNA sequencing¹⁸. This platform involves amultistep approach with generation of a large library of PCR reagentdroplets by a microchip, followed by merging of these reagent dropletswith sample droplets generated from a DNA sample on a second device.These sample-reagent hybrid droplets are then collected in standard PCRtubes for thermocycling, followed by fluorescence detection andsequencing. In this platform, the content of each individual droplet isunknown and is decoded only by offline nucleic acid sequencing.Therefore, it cannot be applied to other applications that requirereal-time detection⁴. A solution to this problem is to associate aunique optical code with each reagent prior to mixing with the sample¹⁹.However, an optical-coding scheme based on fluorescence intensity ispractically limited to a small number of ‘codes’ due to the smallallowable number of fluorophores without spectral crosstalk and thelimited dynamic range of the optical detection setup being used⁴.Furthermore, the electrocoalescence technique used in such platforms fordroplet merging is susceptible to errors of no fusion caused by anexcess of droplets of a reagent or unintended fusion of more than twodroplets due to highly stringent synchronization requirements²⁰. Arecent article demonstrated a pico-injector which can overcome thisproblem and be used to add controlled volumes of multiple reagents tosample droplets using electromicrofluidics²¹. However, similar todroplet platforms discussed earlier, the content of each individualdroplet is unknown unless a barcode is included in each individualdroplet.

Alternatively, a series of articles adopted a cartridge technique forincreasing the throughput of the droplet platform²². This techniqueinvolves generation of an array of reagent plugs in a capillary(cartridge), which are sequentially introduced to a simple microfluidicdevice for merging with a single substrate. The reagent plugs can befurther digitized into smaller droplets prior to merging with thesample. As the length of the capillary can be very long, the number ofreagents to screen against the sample is virtually limitless. Thistechnique has been applied to many applications including proteincrystallization²² and study of bacterial susceptibility toantibiotics²³. Although the aforementioned droplet and cartridgeplatforms are capable of high throughput and multiplexed analysis, theyare still limited to screening of a single sample at a time.

Recently, a microfluidic platform was proposed for combinatorialchemical synthesis in picolitre droplets, where droplets of one libraryof reagents were fused at random with droplets containing a differentset of reagents²⁰. This platform has the potential of generating a largeset of possible combinations of different reagents. However, asafore-discussed, the unknown identity of the compounds within individualdroplets precludes its use for many screening applications that requirereal-time detection.

Some embodiments of the current invention provide a droplet platformcapable of on-demand generation of nanolitre droplets of combinationalmixtures of samples and reagents needed for biochemical screeningapplications that require multiplexing and high-throughput capability.On-demand droplet generation and manipulation using pneumatic valves hasbeen demonstrated by other groups in the past²⁴⁻²⁶. However, theseplatforms have focused on generating multiple reagent combinations usingfixed number of inputs to the device, severely limiting the number ofpossible sample-reagent combinations being generated on the device. Thedroplet platform according to some embodiments of the current inventionuses a linear array of sample plugs as an input to the device, removingthe limitation imposed by the number of inputs to the device. Initially,a preformed linear array of sample plugs separated by a carrier fluid isflowed from the cartridge into the microfluidic device, wherein eachplug is digitized by a pneumatic valve into smaller sample daughterdroplets. The volume of the resulting daughter droplet can be preciselycontrolled by varying the valve opening time and the back pressure onthe cartridge containing sample plugs. The daughter droplets are thendirectly injected with reagents in a synchronization-free manner. Themicrofluidic design features a robust fusion module which exploits localchannel geometry for synchronization-free injection of reagents intoeach sample daughter droplet. After reagent injection into a sampledroplet, a microfluidic device according to some embodiments of thecurrent invention introduces additional carrier fluid containingsurfactant to the channel containing the sample-reagent hybrid dropletarray to prevent unwanted merging of these droplets on the device. In anembodiment of the microfluidic device, droplets are indexed by theirlayout in a 1D array, enabling the identification of the contents ofeach droplet by spatial indexing. Spatial indexing as a means foridentification of droplet content can obviate the need for a limitingoptical barcoding scheme.

FIG. 1 is a schematic illustration of a continuous throughputmicrofluidic system 100 according to an embodiment of the currentinvention. The continuous throughput microfluidic system 100 includes aninput system 102 configured to provide a sequential stream of sampleplugs 104, a droplet generator 106 arranged in fluid connection with theinput system 102 to receive the sequential stream of sample plugs 104.The term “plug” is used here to indicate that it is a larger volume thanthe droplets such that one plug can be used to generate a plurality ofdroplets. The general concepts of the current invention are not limitedby a particular number of droplets produced from each plug. The dropletgenerator 106 is also configured to provide an output stream of droplets(not shown in FIG. 1). The continuous throughput microfluidic system 100also includes a droplet treatment system 110 arranged in fluidconnection with the droplet generator 106 to receive the output streamof droplets in a sequential order. The droplet generator 106 is alsoconfigured to provide a stream of treated droplets 108 in the sequentialorder, a few of which are illustrated in FIG. 1. The continuousthroughput microfluidic system 100 also includes a detection system 112arranged to obtain detection signals from the treated droplets in thesequential order. The continuous throughput microfluidic system 100further includes a control system 114 configured to communicate with theinput system 102, the droplet generator 106, and the droplet treatmentsystem 110. The continuous throughput microfluidic system 100 alsoincludes a data processing and storage system 116 configured tocommunicate with the control system 114 and the detection system 112.

The data processing and storage system 116 can be, but is not limitedto, a programming computer, for example. The computer can be a localizedcomputer, such as, but not limited to, a lap top computer, a desk topcomputer, or a workstation. However, the computer can also be adistributed system, such as a networked system of computers. The controlsystem 114 can similarly have programming components implemented on thesame of different computer as data processing and storage system 116.The data processing and storage system 116 and/or control system 114 canalso include hard wired electronic components in addition to, or insteadof software implemented functions.

The control system 114 is configured to control the input system 102 inconjunction with the droplet generator 106 and to provide information tothe data processing and storage system 116 that identifies each dropletof the output stream of droplets with a corresponding sample plug ofsaid sequential stream of sample plugs 104. The control system 114further controls the droplet treatment system 110 and providesinformation to the data processing and storage system 116 thatidentifies a treatment applied to each droplet of the output stream ofdroplets. The data processing and storage system 116 receives thedetection signals and calculates a property of each treated droplet andidentifies a corresponding plug and treatment for each treated dropletbased on the sequential order.

In some embodiments continuous throughput microfluidic system 100 caninclude a microfluidic chip 118 that defines a microfluidic channel 120that includes an input end 122 configured to be fluidly connected to theinput system 102. As illustrated in the example of FIG. 1, microfluidicchannel 120 can have a first segment that is integrated with the dropletgenerator 106, a second segment that is integrated with the droplettreatment system 110, and a third segment that provides a measurementregion 124 for the detection system 112. The term “microfluidic” channelmeans that the channel has cross-sectional dimensions that are less thanone millimeter. For example, for an approximately circularcross-sectional channel, the channel diameter is less than onemillimeter. For a square or rectangular cross-sectional channel, thechannel height and width are both less than one millimeter. In someembodiments. The channel cross-sectional dimensions can be tens ofmicrons, a few microns, or even less than one micron.

The microfluidic chip 118 can also be a multilayer microfluidic chip insome embodiments of the current invention. For example, the microfluidicchip 118 can include one or more channel layers, and one or more controllayers to hydraulically and/or pneumatically control valves that can beincorporated with the microfluidic chip 118. The microfluidic chip 118can have multiple sections for performing multiple functions, as isillustrated in FIG. 1. However, other embodiments could combine two ormore microfluidic chips for performing the desired functions withoutdeparting from the broad concepts of the current invention.

The microfluidic chip 118 in the embodiment of FIG. 1 further defines avalve assembly 125 as a component of the droplet generator 106 that isselectively controllable by the control system 114. The dropletgenerator 106 also includes a fluid channel 126 for receiving carrierfluid to be input into the microfluidic channel 120 between adjacentdroplets. The treatment system 110 can include a reagent adding section128 and at least one of a reaction section or an incubation section 130,for example. The treatment system 110 can include additional and/oralternative functional units than those described in the example of FIG.1.

The reagent adding section 128 includes a plurality of reagent inputchannels and corresponding valve assemblies configured to communicatewith the control system such that one or more reagents can beselectively added to a selected droplet when the droplet is in thesecond segment of the microfluidic channel within the reagent addingsection at a selected time. The example of FIG. 1 shows four reagentchannels and corresponding valve assemblies. However, the broad conceptsof the invention are not limited to a particular number of reagentchannels. Alternatively, one, two or three could be used, or more thanfour could be used. In some cases, many more than four can be used. Forexample, tens or even hundreds could be included.

The reagent adding section 128 includes a portion of the microfluidicchannel 120 in which a cross-section area is decreased relative toadjacent sections to stretch droplets across at least some of theplurality of reagent input channels. Some examples of a dropletstretching segment of the channel are described in more detail below.The reagent adding section 128 further includes a stabilizing-fluidinput channel 132 and a corresponding valve assembly configured tocommunicate with the control system 114 such that a stabilizing fluidcontaining a surfactant can be selectively added to droplets within thedroplet treatment system. The stabilizing fluid can be a carrier fluidsuch as that used to separate the plugs and droplets with the additionof a surfactant, for example. The stabilizing-fluid input channel 132 isarranged downstream from the plurality of reagent input channels 128such that droplets can be stabilized after addition of reagent. In manyapplications, one of the plurality of reagents will be added. However,the broad concepts of the current invention are not limited to thatexample. Any combination of two or more of the available reagents couldbe added, if desired for a particular application.

In some embodiments, the reaction section or incubation section 130includes a temperature control component. For example, the shaded areacould be a Peltier component to heat and/or cool the reaction orincubation section. Other temperature control components could also beused, such as, but not limited to, resistive heating elements and/orheat conduction component's that are in thermal contact with externalheat sources or heat sinks. The temperature control component can alsobe adapted to communicate with the control system. This can permitmaintaining a constant temperature and/or providing a programedtemperature profile, either spatially along the microfluidic channel 120and/or changing with time. Furthermore the microfluidic channel can havea serpentine path in the reaction section or incubation section 130 toallow for a compact arrangement with an extended path length. In someembodiments, as in some examples described in more detail below, thereaction section or incubation section 130 is an incubation section.

In some embodiments, the input system 102 includes a capillary tube 104that is suitable to be loaded with the sequential stream of sample plugswith separation fluid between adjacent sample plugs. For example, asilica capillary tube could be used in some embodiments. However, thebroad concepts of the current invention are not limited to this example.For example, without limitation, other cartridge or tube structurescould be used.

As will be described in more detail below in reference to particularexamples, capillary tube 104 of the input system 102 has across-sectional opening that extends beyond a cross-sectional opening ofthe input end 122 of the microfluidic channel 120. The input system 102can further include an adapter that has a first end that substantiallymatches the cross-sectional opening of the capillary tube 104 and asecond end that substantially matches the cross-sectional opening of theinput end 122 of the microfluidic channel 120. In some embodiments, theadapter has a substantially smooth inner surface that tapers from thefirst end to the second end. In alternative embodiments, the adapter hasa segmented inner surface that provides a plurality of steps totransition from the first end to the second end.

In some embodiments, the input system 102 can further include anautomated sample loader 134 configured to load the capillary tube 104with the sequential stream of sample plugs and with carrier fluidbetween adjacent sample plugs from a multi well plate and to deliver andfluidly connect the capillary tube and the adapter to the input end 122of the microfluidic channel 120. In some embodiments, the automatedsample loader 134 can have a linear three-axis stage to move the multiwell plate while maintaining the capillary tube 104 fixed. Moving themulti well plate instead of the capillary tube 104 can be advantageousin some applications to prevent disturbing sample as it is being loaded.However, the capillary tube 104 could be moved instead of, or inaddition to, the multi well plate in some embodiments.

The detection system 112 can be, or can include, an optical system.However, other embodiments can include additional or alternatives tooptical systems. The optical system can be, but is not limited to, afluorescence spectroscopy system.

The control system 114 can be configured to selectively, start, stop andregulate a flow speed of the output stream of droplets and the stream oftreated droplets, for example.

In operation of the continuous throughput microfluidic system 100, theinput system 102 loads a sequential stream of sample plugs 104 in aparticular order as regulated and recorded by the control system 114 andthe data processing and storage system. The droplet generator 106receives the sequential stream of sample plugs 104 from the input system102 and generates a plurality of droplets from each plug to provide astream of droplets ordered in accordance with the order generated fromthe respective plug. (FIG. 2 provides a schematic illustrationdescribing operation the system 100.) This sequence, thus provide alinear array of droplets which can then be processed in a manneranalogous to an assembly line. The identities and subsequent processingof each droplet is known as corresponding to the place within the lineararray. For example, the first droplet generated, will be the firstdroplet measured at the end; and the one-hundredth droplet generatedwill be the one-hundredth droplet measured and the end. The processingalong the way is also known. For example when one or more reagents areadded to a particular droplet in reagent adding section 128, the dropletenters a constricted region to stretch the droplet across multiplereagent channels. For example, the droplet can be stretched to extendacross all reagent channels in some embodiments. The control system canthen stop the “assembly line” such that the stretch droplet is held fora desired period of time in which to effect the addition of the one ormore reagents. The “assembly line” can then be restarted to bringanother droplet into the constricted region to be stretched across thereagent channels. This process can be continue in a “continuous” manner.The term “continuous” is intended to mean that there is not a finitespecific number of droplets that can be processed. The assembly line canbe started and run as long as is desired in a particular application. Inaddition, the droplets can be generated and processed on demand undercontrol of the control system 114. In addition, once the assembly lineis started, the system has very high throughput since it is a continuousstream.

Although the embodiment described above has one microfluidic channel120, other embodiments can include multiple systems operating inparallel to further increase throughput. Since the system is amicrofluidic system, some embodiments can include large numbers of suchsystems in parallel. For example, there could be tens, hundreds, or eventhousands of such systems operating in parallel.

The following examples describe some embodiments and some applicationsin more detail. However, the broad concepts of the current invention arenot limited to the particular examples.

EXAMPLES

Materials and Methods

Serial Sampling Loading System

The sample library was generated using a custom-designed Serial SampleLoading System (SSL). FIG. 3A is a schematic illustrating thefunctioning of the SSL system. Briefly, the SSL system was designed tobe compatible with Costar 96-well plates (Corning). Initially the wellson a Costar 96-well plate are filled with the samples and the carrierfluid to be used for generating the sample plug array. An Aquapel (PPGIndustries) treated silica capillary is then attached to a capillaryadapter on the SSL system, which is also connected to a positivepressure input. A sample well is then interfaced with the capillarythrough the capillary adapter. Application of positive pressure to thesealed sample well for a controlled amount of time is then used to drivea sample plug from the well into the capillary. This sequence of stepsis then repeated to load alternating sample and carrier fluid plugs intothe capillary (FIG. 3B). More detailed information on the structure andoperation of different components of the SSL system can be found below.

Fabrication of the Master Molds for the Microfluidic Device

The fluidic layer on the microfluidic device features five differentheights of microfluidic channels (FIG. 4B). As a result, the fluidicmold consists of five different layers of photoresist. The fluidicchannel heights in these five different photoresist layers were expectedto be 25 μm, 50 μm, 100 μm, 200 μm and 360 μm. The photoresist used forthe 25 μm layer was SPR 220-7.0 (Rohm & Haas), while the rest of thelayers were fabricated using SU-8 3050 (MicroChem). Fabrication wasperformed using standard photolithography techniques. Briefly, a SPR220-7.0 layer was spin coated on a 4 inch silicon wafer. This layer waspatterned using photolithography and hard baked to generate a roundedchannel cross section, required for effective valve closure, as has beendescribed earlier²⁷. For all other layers, SU-8 3050 was spin-coated onthe wafer and patterned using standard photolithography, excluding thedeveloping step. This technique was found to be very effective inpreventing generation of bubbles and non-uniform coating of photoresiston the wafer due to the presence of features from earlier layers on thewafer. A single developing step for all four SU-8 3050 layers was usedto remove excess photoresist on the wafer (FIG. 8). The control layerfor the microfluidic device on the other hand consisted of microfluidicchannels of a single height. As a result, the mold fabrication for thecontrol layer was relatively simpler, with a, single layer of SU-8 3050photoresist, 50 μm in height.

Microfluidic Device Fabrication

The microfluidic devices were fabricated using multilayer softlithography techniques²⁷. The protocol differed slightly from ourstandard protocol²⁸⁻³⁰ due to the need for proper functioning ofpush-down valves³¹ while accommodating tall features (up to 360 μm) onthe fluidic layer. The thickness of the polydimethylsiloxane (PDMS)membrane separating the control layer and the fluidic layer in amicrofluidic device needs to be less than ˜50 μm for complete valveclosure at reasonable pressure (˜30 PSI). However, the presence offluidic regions as tall as 360 μm on the fluidic layer mold precludedthe possibility of covering the entire fluidic layer mold with PDMS,while maintaining the thickness of the PDMS layer to a value less than50 μm in the regions of the device containing valves. To overcome thisproblem, a modified three-layer fabrication process was developed.Detailed description of the fabrication process is included below (FIG.9).

Capillary-to-Chip Interface

Following the microfluidic device fabrication, a silica capillary wasattached to the ‘capillary inlet’ on the microfluidic device (FIG. 4B).The 360 μm tall channel region at the capillary inlet accommodates asilica capillary with an OD of 360 μm. A 10 mm section of silicacapillary is inserted horizontally into this tall channel on the deviceuntil it is flush with the 200 μm tall fluidic channel on the device. Toseal the capillary to the chip and prevent leakage, PDMS was dispensedaround the capillary at the interface between the capillary and thedevice. The PDMS tended to crawl into the 360 μm channel and surroundthe capillary, effectively sealing the capillary-to-chip connection. Thefinal assembly was baked for at least 2 hours at 80° C. before usage.

Device Control

All the inputs on the device were kept under constant pressure, withindependent input pressure for 1) carrier fluid input, 2) all fourreagent inputs and 3) carrier fluid with surfactant input. The pressureapplied to the capillary input was controlled directly by the pressurecontroller used for the SSL system. All the valves on the device werecontrolled by an array of off-chip solenoid valves, as has beendemonstrated earlier²⁸. We developed Matlab (Mathworks, Natick Mass.)software for computer control of the valve array. This software allowedus to execute a predetermined sequence of valve actuation withindependent time control for each actuation. The opening of a valvecorresponding to an input on the device led to the release of a dropletof fluid from that inlet into a central channel on the device. Thevolume of this droplet could be controlled through variation of theopening time of the valve.

Reagents

All the devices and capillaries were treated with Aquapel to rendertheir surface hydrophobic. The testing of our platform was performedusing food dyes (Ateco, Glen Cove, N.Y.) to mimic different samples andreagents for easy visualization. The carrier fluid used to maintain theseparation between sample plugs consisted of a perfluorocarbon (FC-3283)and a non-ionic fluorous-soluble surfactant(1H,1H,2H,2H-Perfluoro-1-octanol) mixed in a ratio of 8:1 by volume. Thecarrier fluid with surfactant consisted of FC-40 (3M) and 2% ‘EA’surfactant (Raindance Technologies) by weight.

Sample Plug and Droplet Volume Estimation

We estimated the volume of sample plugs and sample droplets generatedusing the SSL system and the microfluidic device respectively. Thisvolume estimation was performed by processing the images of these sampleplugs or droplets using the software ImageJ³². Specifically, for sampleplug volume estimation, a series of sample (blue food dye) plugs weregenerated in a silica capillary using the SSL system. A color image ofthese plugs was taken against the white background of a ‘letter’ sizedsheet of paper using a standard Digital Single-Lens Reflex (DSLR)camera. This image was imported in ImageJ and the length scale was setto true length using the known length of the letter sized paper in theimage. The lengths of the sample plugs were then manually measured foreach plug using the ‘Measure’ function in ImageJ. The plug lengths couldbe converted to plug volumes with the known cross sectional area of thecapillary.

For sample droplet volume estimation, we generated droplets made of bluefood dye using one of the four reagent inlets on the microfluidicdevice, until the whole incubation region on the device was full ofdroplets. The whole device was then imaged using a DSLR camera. Theimage was imported in ImageJ and cropped to obtain an image of theincubation region on the device. This image was then converted to abinary image using color thresholding to identify droplets over thebackground image. An estimate of the droplet area for each droplet inthe image was then obtained using the ‘Analyze Particles’ function. Thisanalysis was limited to particle areas larger than a lower threshold toexclude any particles and occasional satellite droplets from theanalysis. The droplet areas thus estimated were then converted todroplet volume using the known depth of the incubation channel region(200 μm).

Results and Discussion

Overall Work Flow

FIG. 2 is a schematic illustrating the functioning of the platform.Initially a cartridge (capillary) is loaded with a library of sampleplugs forming a serial sample plug array: plugs are separated from eachother by an immiscible carrier fluid. This cartridge is interfaced witha microfluidic device featuring multichannel architecture and pneumaticmicrovalves. The microfluidic device digitizes sample plugs into smallerdaughter droplets. Each sample daughter droplet then moves to thedownstream fusion region where a specific reagent is injected into thesample daughter droplet. The reagent droplets are injected into thesample daughter droplet through controlled actuation of valvescorresponding to the reagent inlets. No strict synchronization ordroplet detection module is necessary for fusion of sample and reagentto occur as the sample droplet is elongated in the fusion areaexploiting the local channel geometry. The resulting sample-reagentdroplet undergoes mixing and travels downstream to the incubation regionon the device. After reagent injection, additional carrier fluidcontaining surfactant is released into the central channel on the deviceto stabilize sample-reagent hybrid droplets. The sequence of droplets ismaintained throughout the device, precluding the need for a complicatedbarcoding scheme to identify the contents of each individual droplet.

Capillary-to-Chip Interface

Our prototype platform necessitated the capillary-to-chip interfacedesign to allow for sample plug introduction on chip. This objectivepresented a unique challenge, since proper functioning of the platformrequires smooth transition of sample plugs from the large ID of thecapillary to shallow channels on the device in the valve regions. Therehave been demonstrations of capillary-to-chip interfaces in the past forintroducing sample plugs from a capillary to a microfluidic device.However, the devices used don't face this problem as they typicallyfeature large channels with a valveless design^(22,23). The capillaryinterface we designed (FIG. 4B) between the capillary and microfluidicchip was found to be effective in minimizing plug break up as plugsmoved from the high ID (200 μm)μm) of the capillary to the shallowchannels on chip (25 μm). This transition consisted of 5 differentchannel sections with gradually reducing channel heights of 360 μm, 200μm, 100 μm, 50 μm and 25 μm. This gradual transition minimizes the shearstress on the sample plug as it traverses from a capillary to theshallow channels on the chip, preventing its breakup in transit.

Droplet Uniformity Using Mechanical Valve Based Droplet Generation

We examined the performance of the mechanical valves on our microfluidicdevice for their capability to control the droplet size generated. Toconduct this experiment, we primed the incubation channel on the devicewith the carrier fluid. We then used one of the reagent inlets on thedevice for generating droplets made of blue-colored food dye into theincubation channel region. The two parameters which could be used tocontrol the droplet size generated from a reagent inlet, are 1) Inputpressure to the reagent inlet (P_(reagent)) and 2) The opening time ofthe valve corresponding to the reagent inlet (T_(open)). Initially, wefixed the value of P_(reagent) and generated droplets on the device fordifferent values of T_(open). Droplet generation was continued for eachcondition tested until the incubation region on the device wascompletely full of droplets. We then estimated the volume for all thesedroplets using the image processing technique discussed in the‘Materials and Methods’ section. The mean and standard deviation offifty droplets generated for each condition was plotted against T_(open)in FIG. 5B. This experiment was repeated for three different fixedvalues of P_(reagent). As expected, the linear relationship betweendroplet volume and T_(open) indicates excellent and predictable controlof the device over droplet volume. Small standard deviation observed onthe droplet volume also indicates excellent droplet uniformity foridentical droplet generation conditions. This result is very importantto ensure the capability of the device to generate droplets of variouscompositions on-demand.

Sample Digitization

We examined the capability of our device to digitize a set of sampleplugs being supplied to the device into smaller sample daughterdroplets. To conduct this experiment, we generated a set of sample plugsinto a silica capillary using the SSL system. These sample plugs weredelivered to the microfluidic device through the capillary inlet, underpressure provided by the pressure controller on the SSL system. For thisexperiment, the repeating sequence of steps executed on the device wasas follows: 1) Generate small droplet from a sample plug in the centralchannel, 2) Move the droplet towards incubation region with carrierfluid 3) Release small amount of carrier fluid with surfactant in thecentral channel. Repeating this set of steps led to generation of anarray of sample droplets generated through digitization of sample plugson the device. Examples of unmerged sample daughter droplets are shownin FIG. 7A (Sample droplets A, B, C and D). The order of the sampleplugs in the capillary is consistently maintained on the device, evenafter the digitization operation. One shortcoming of this operation isthe generation of non-uniform droplets towards the beginning and the endof the sample plugs. This is because the valve actuation sequence iscontinuously executed without any sensing of sample plug arrival on thedevice. However, the sample droplet uniformity is maintained throughoutthe rest of the sample plug. As the droplets generated on the device arestabilized with surfactant, undesirable merging of non-uniform dropletsoriginating from the front- or back-end of plugs is avoided on thedevice.

Generation of Droplets of Combinatorial Mixtures

In this section we demonstrate an example of generation of combinatorialmixtures from sample plugs and reagent droplets on our device accordingto an embodiment of the current invention. For discernibility, we choseto use different food dye solutions to simulate different samples andreagents. FIGS. 6A and 6B show how reagent injection operations areperformed in the fusion region on an embodiment of our device. First, asample plug travels from the capillary on to the microfluidic device.This plug is then chopped into a smaller sample daughter droplet. Thisdroplet is then moved to the downstream fusion zone through release ofcarrier fluid in the central channel on the device. As every singleinput on the device is controlled with an individual valve, the devicefunctions like an assembly line with complete temporal and spatialcontrol over every single operation, as against typical dropletgenerating devices where the carried fluid flow is continuous. Thislevel of control also implies that the operation of the device can bepaused and resumed with a completely new valve actuation sequence ondemand without affecting the existing droplets on the device. None ofthe droplet devices reported in literature so far has this capability tothe best of our knowledge. A reagent droplet is then injected directlyinto the sample daughter droplet (FIG. 6A). The volume of reagentsolution injected into the sample droplet can be controlled throughvariation of the opening time for the valve corresponding to the reagentinlet. The fusion zone is designed such that the sample daughter dropletis sufficiently elongated within a region, which overlaps with all theinjection ports of the interrogating reagents. This elongated dropletstate removes the need for strict positioning accuracy requirements onthe sample droplet for reliable injection of reagent into the sampledroplet. In addition to demonstrating injection of a single reagent in asample droplet, we have demonstrated injection of up to four reagentsinto a single sample droplet as shown in FIG. 6B. The concept of dropletelongation to aid reagent injection can be easily scaled to accommodatetens of reagent inlets, if desired. Post reagent injection, no unwantedmixing of reagents was observed with subsequent sample plugs. Occasionalresidual reagents residing between the activated valve and centralchannel at a reagent inlet are encapsulated by a sheath of carrier fluidwhich prevents fusion with the next sample droplet.

After reagent injection, the sample-reagent droplet is driven furtherdownstream with the help of carrier fluid. Following this, a small plugof carrier fluid with surfactant is released in the central channel forstabilizing the droplets in the incubation region. Using this scheme wecan simultaneously take advantage of a surfactant-free zone in one areaof the chip to promote sample-reagent merging while deliberately usingsurfactant in another area to increase droplet stability and preventunwanted droplet merging. In addition, the backpressure on the carrierfluid inlets was used to control flow velocity of the droplets. For theresults presented in this paper, the flow velocity of droplets was ˜5mm/second. However, the flow velocity can be easily tuned by controllingthe back pressure on the central carrier fluid channel.

FIGS. 7A and 7B demonstrate the reliability of the fusion mechanism onour device. FIG. 7A is a table of 16 different sample-reagentcombinations generated on a single chip through all possible mergingcombinations of four different sample daughter droplets (A: blue, B:yellow, C: green, D: water) with four different reagents (Reagent 1:orange, Reagent 2: water; Reagent 3: blue, Reagent 4: yellow) with thecondition of merging exactly one sample with one reagent.

The micrographs in FIG. 7B show a repeating sequence of thesample-reagent hybrid droplets in the incubation region of the chip. Thechip is operated such that the sample daughter droplets are merged witha repeating sequence of four different reagents. As a result a repeatingsequence of four possible combinations generated through mixing a singlesample with four different reagents can be seen in each individualmicrograph. Once a sample plug is exhausted, the sample daughterdroplets generated from the next incoming sample plug start merging withthe same repeating sequence of reagents generating a repeating sequenceof a new set of four different sample-reagent combinations in theincubation region on the chip.

The droplet monodispersity as well as the uniform spacing betweendroplets is clearly visible in these micrographs. The inset in FIG. 7Bdisplays zoomed-in view of these micrographs of the incubation regionillustrating two repeats of each sequence in the incubation region.These images also demonstrate the capability of the device to maintainthe order in which droplets are generated throughout the incubationregion on the device. We have demonstrated 16 combinations in thisinstance, but by employing multiple (2, 3 or 4) reagent merging withsample daughter droplets, as demonstrated in FIG. 6B, many morecombinations can be generated using our device.

Mold Fabrication

The work flow used for fabricating the fluidic layer mold is illustratedin FIG. 8. The mold consists of five different layers of photoresistwith heights of 25 μm, 50 μm, 100 μm, 200 μm and 360 μm. The photoresistused for the 25 μm layer was SPR 220-7.0 (Rohm & Haas), while the restof the layers were fabricated using SU-8 3050 (MicroChem). Initially a25 μm tall SPR 220-7.0 layer was spin coated on a silicon wafer. Thislayer was patterned using photolithography and hard baked to generate arounded channel cross section, required for effective valve closure. Therest of the layers were fabricated by stacking and patterning multiplelayers of SU-8 3050 on the wafer. All of the steps required for standardphotolithography (Soft Bake, Exposure and Post Exposure Bake) areconducted for each layer of SU-8 3050, except for the developing step.This step is conducted in common for all layers after the last SU-8layer is patterned to remove excess unexposed photoresist from the wafer(FIG. 8). This technique was found to be very effective in preventinggeneration of bubbles and non-uniform coating of photoresist on thewafer due to the presence of features from earlier layers on the wafer.

Device Fabrication and Operation

The microfluidic device for our experiments was fabricated usingmultilayer soft lithography technique. Standard dual layer microfluidicdevices with push-down valves fabricated using polydimethylsiloxane(PDMS) require shallow fluidic channels to make sure the layer of PDMSbetween the fluidic and control layer is sufficiently thin (˜50 μm) forcomplete closure of valves at low pressures (<30 psi). The requirementof shallow fluidic channels is incompatible with our chip design. So wedeveloped a modified fabrication process for our device. This modifiedsoft lithography process is outlined in FIG. 9. For this modifiedfabrication process, three different batches of PDMS were mixed. Thesevaried in composition, and base to crosslinking agent ratios of 15:1,10:1 and 6:1 were used, respectively. These batches were thoroughlymixed and degassed prior to use for device fabrication. The controllayer mold was spin coated with a thick layer (˜1 mm) of 6:1 PDMS andbaked at 80° C. for 7 mins. A thin layer of 15:1 PDMS was spin coated onthe fluidic layer mold. The device was designed such that the valveregions on the device were placed in areas surrounded by shallow fluidicchannels, ensuring uniform coverage of these regions with a thin layerof PDMS. The PDMS on the fluidic layer mold was then baked at 80° C. for6 minutes. The PDMS was removed from the control layer mold and thecontrol layer was cut to the exact size of the valve regions on thedevice, while not covering any channels higher than 50 μm on the device(FIG. 9). The control layer PDMS pieces were aligned with baked PDMSlayer on the fluidic layer mold under a stereoscope. The fluidic layermold with the aligned control layer was baked at 80° C. for 20 mins topromote adhesion between the control layer and the fluidic layer.Following this, 49.5 g of 10:1 PDMS was poured on the fluidic layermold, covering all features on the fluidic layer mold with a 3-4 mmthick layer of PDMS. The fluidic layer mold was then baked for at least30 minutes at 80° C. Following this, the PDMS was removed from thefluidic layer mold and individual devices were cut. Fluidic access holeswere then punched into the device and the device was bonded to acoverglass through oxygen plasma treatment.

Fusion Zone Design

An important aspect of an embodiment of our microfluidic device is therobust synchronization-free fusion mechanism. This mechanism utilizesthe cross-sectional area of the central channel on the microfluidicdevice for the merging operation. FIG. 10 demonstrates the designcriteria of the fusion region of an embodiment of our device. In thecurrent example, the dimensions of the fusion zone, defined as thedistance between the first and last reagent inlet (3800 μm), height (50μm) and width (100 μm) of the central channel determine the volume ofthe fusion zone (19 nL). This volume of the fusion zone corresponds tothe minimum volume of the sample daughter droplet, such that the dropletspans the entire length of all the reagent injection sites on the chip.As a result, the sample daughter droplet position doesn't need to befinely controlled to inject different reagents in it. If there is a needfor smaller reaction volumes, the cross-sectional area of the centralchannel can be modified to reduce the minimum volume of the sampledaughter droplet required. For instance, reducing the fusion zonechannel height from 50 μm to 10 μm will results in reduction in minimumrequired droplet volume from 19 nL to 3.8 nL (FIG. 10). A similarapproach can be used to accommodate more than four reagent inlets on thechip.

Serial Sample Loading System

FIG. 11 shows an actual assembled Serial Sample Loading (SSL) system forsample library generation according to an embodiment of the currentinvention. The SSL system consists of 1) a custom-made capillaryadapter, 2) an automated Z-stage, 3) manual X- and Y-stages 4) amulti-well plate and 5) an Electronic Pressure Controller. The capillaryadapter was designed in Solidworks (Solidworks Corp.) and thenfabricated by the staff at the Physical Sciences Machine Shop at JohnsHopkins University. A motorized Lab Jack (L490MZ/M, Thorlabs) was usedas the automated Z-stage. The manual X and Y stages were purchased fromMelles Griot(Albuquerque, N. Mex.). In its current form, the SSL systemwas designed to be compatible with the Costar 96-well plates (Corning).The Electronic Pressure Controller (PCD-100PSIG-D-PCV03, AlicatScientific) used in the SSL system is a dual valve pressure controllerdesigned for pressure control in a closed volume. All other structuralcomponents of the SSL system were purchased from Thorlabs (Newton,N.J.). Custom software developed in LabVIEW was used to control theZ-motion of the automated Z-stage and the injection pressure applied bythe Electronic Pressure Controller.

The capillary adapter in the SSL system features three different ports,which are designed for accepting a microcapillary input, a pressureinput and an output for gauging pressure inside a sealed sample well. Weattached NanoPorts (Idex Health and Science) at these three ports forconsistent leak free connections with tubing corresponding to each port.The bottom surface of the capillary adapter also holds a siliconesealing ring, fabricated from Silicone Septa (1395-32SS, Corning) usedto seal a sample well with the capillary adapter. All the three ports onthe capillary adapter are routed to the bottom surface of the capillaryadapter where they open into a sealed sample well. For most of ourexperiments, a silica microcapillary (360 μm OD and 200 μm ID) wasattached to the capillary input of the capillary adapter, unlessspecified otherwise. The pressure input was connected to the output ofthe pressure controller. The pressure gauge port was unused and keptplugged for all the experiments. Prior to use, the silica microcapillaryis treated with Aquapel™ (PPG Industries).

Conclusion

In the above examples, we have demonstrated a platform capable ofpreparing droplets from combinational mixtures of a large number ofsamples and reagents. This is accomplished by synchronization-free anddetection-free fusion of sample daughter droplets and reagents. Abenefit of this architecture can include the ability to scale thisdevice to analyze N samples against M reagents (N×M) where N can rangefrom hundreds to thousands without accompanying increase in devicecomplexity. Additional reagent set multiplexing can be accomplishedanalogously by introducing linear arrays of reagent set plugs similar tosample introduction. Furthermore, this design allows for spatialindexing, by maintaining the sequence of droplets from generationthroughout incubation, precluding the need for barcoding.

Some components can include: a unique SSL system which uses pressure toinject uniform volumes of sample into a capillary directly from anindustry standard multi-well plate. This capillary is then interfacedwith a microfluidic device using a novel capillary-to-chip connection.The microfluidic device is capable of combinatorial screeningoperations. Robust synchronization-free reagent injection is performedon the device based on a design which capitalizes on droplet elongationin the fusion zone on the device. In an embodiment, up to 4 reagentdroplets can be fused with a single sample droplet. However, byemploying the same concept many more reagent inlets can be introduced onchip to perform merging operations. In addition, we have demonstrated atechnique for reagent injection in droplets that capitalizes oncontrolling droplet surface chemistry by controlling surfactantconcentration at different regions on the chip. That is, we havedemonstrated a surfactant-free environment in the fusion zone on thedevice, thereby promoting reagent injection in sample droplets while thedroplets are stabilized by surfactant in the incubation region.

For the microfluidic chip design, several areas can be explored tofurther enhance the operation of the chip. To make the transition ofsample plugs from a capillary to the microfluidic device more gradual aphotolithography process employing a grayscale mask could be used. Thisapproach can generate very gradual reduction in channel cross sectionfrom a large capillary to shallow microfluidic channels on the devicecompared to the step reduction demonstrated in the example above.Furthermore, reagents may be loaded in cartridge format to furtherenhance multiplexing capabilities. We expect the platform described hereto be a promising candidate for combinatorial screening applicationsusing droplet microfluidics.

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The embodiments illustrated and discussed in this specification areintended only to teach those skilled in the art how to make and use theinvention. In describing embodiments of the invention, specificterminology is employed for the sake of clarity. However, the inventionis not intended to be limited to the specific terminology so selected.The above-described embodiments of the invention may be modified orvaried, without departing from the invention, as appreciated by thoseskilled in the art in light of the above teachings. It is therefore tobe understood that, within the scope of the claims and theirequivalents, the invention may be practiced otherwise than asspecifically described.

1-17. (canceled)
 18. A method of screening a plurality of samples, comprising: obtaining a sequential stream of sample plugs; generating a sequential stream of droplets from said sequential stream of sample plugs using a selectively controllable microfluidic system; adding at least one reagent from a plurality of reagents to each of said sequential stream of droplets in sequential order using said selectively controllable microfluidic system; and measuring at least one physical property of each of said sequential stream of droplets in said sequential order, wherein information concerning an identity of said plug from which each droplet of said sequential stream of droplets is generated and said at least one reagent added to each droplet is used to identify measured droplets based on said sequential order during said measuring.
 19. A method of screening a plurality of samples according to claim 18, further comprising adding a stabilizer to each of said sequential stream of droplets after said adding at least one reagent to help prevent adjacent droplets from coalescing.
 20. A method of screening a plurality of samples according to claim 18, further comprising regulating a temperature of an incubation section of said selectively controllable microfluidic system after said adding at least one reagent and prior to said measuring.
 21. A method of screening a plurality of samples according to claim 18, wherein said obtaining a sequential stream of sample plugs comprises obtaining at least one capillary tube that is loaded with sequential stream of sample plugs with a carrier fluid between adjacent sample plugs.
 22. A method of screening a plurality of samples according to claim 21, wherein said obtaining a sequential stream of sample plugs comprises loading said at least one capillary from a multi well plate.
 23. A method of screening a plurality of samples according to claim 21, wherein said loaded using an automated loading system.
 24. A continuous throughput microfluidic system, comprising: an input system configured to provide a sequential stream of sample plugs; a droplet generator arranged in fluid connection with said input system to receive said sequential stream of sample plugs and configured to provide an output stream of droplets; a droplet treatment system arranged in fluid connection with said droplet generator to receive said output stream of droplets in a sequential order and configured to provide a stream of treated droplets in said sequential order; a detection system arranged to obtain detection signals from said treated droplets in said sequential order; a control system configured to communicate with said input system, said droplet generator, and said droplet treatment system; and a data processing and storage system configured to communicate with said control system and said detection system. 